@Dave Jeffery - your issue could be a known bug in cairo 1.10 when exporting objects with opaque gradients to PDF:Bug #681512 “Opaque radial gradients cause PDFs to fail to open in Adobe Reader”Bug #672686 “Some diagrams results in "Illegal character" in the generated pdf”

Using PDF 1.5 is unlikely to be causing issues with Adobe Acrobat. PDF is designed to be forward compatible. ie if the PDF file contains features that the reader does not know about, those features are ignored while the rest of the file displays correctly. The PDF 1.5 features that cairo supports are not used by Inkscape so even though the version in the file is 1.5 it is only using 1.4 features.

There are many other differences between 0.47 and 0.48 that could be the cause of your bug. Usually problems with opening PDF files are a cairo bug since cairo should never generate an invalid PDF file (unless cairo returns an error status).